The present invention relates to a method of and apparatus for measuring the opacity of sheet material, and in particular to a method and apparatus wherein the opacity of sheet material is determined as a function of the light transmission characteristics of the material.
In the manufacture of lightweight specialty packaging papers of the dry opaque and saturated waxed grades, such as may be used for cereal box overwraps, gum wrappers, paper/poly/foil/poly pouches, antiwater wicking labels, orange juice spiral canisters, potato chip bags, gift wrappings, etc., an important area of quality control resides in the ability to manufacture such packaging papers to have specified opaque grade structures. The required degree of opacity in such papers is developed through the use of titanium dioxide and controlled through a series of rigid manufacturing specifications.
As a result of customer demands on the paper industry, grade specifications of papers and the quantity and cost of titanium dioxide, there exists a need to accurately measure opacity of paper products of both the dry opaque and saturated waxed grades. As is known, opacity is a fundamental optical property of paper as a whole, yet measurement of opacity is empirical. The opacity of a sheet of paper is influenced by the amounts and kinds of fillers, the degree of bleaching of the fibers, types of coating materials applied to the papers and, when used, the thickness of the wax. Throughout the paper industry as a whole, opacity has heretofore been evaluated as a function of contrast ratio measurements, generally following TAPPI Procedure T-425, which evaluation procedure is based upon a ratio of reflectances over a black background to those over a white background.
Instruments for measuring the opacity of paper as a ratio of reflectances are well known and manufactured by Bausch & Lomb and others. Unfortunately, in use of such instruments the calibration procedure required before each test reading is very time consuming, and the area of the paper sample which may be evaluated by an individual measurement is quite small and on the order of 0.625" in diameter. As a result, production personnel often do not have sufficient time to properly and thoroughly evaluate a CD (cross direction) profile test sample due to the tedious calibration procedure required and the limited size of the individual areas of the sample which may be evaluated.
Considering the tedious calibration procedure required in use of conventional paper opacity measuring instruments, and the relatively small areas of samples which may be evaluated, optical opacity evaluation by contrast ratio measurements is not a practical procedure.